Method and apparatus for antenna calibration in a wireless communication system

ABSTRACT

A method is implemented in a digital unit connected with a plurality of distributed antennas including a first antenna, a second antenna and a third antenna. The method comprises: causing transmitting a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource; obtaining a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna; and obtaining estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained loop-back phase difference.

TECHNICAL FIELD

The non-limiting and example embodiments of the present disclosuregenerally relate to a technical field of wireless communication, andspecifically to methods, apparatuses and computer program products forantenna calibration in a distributed antenna system (DAS).

BACKGROUND

This section introduces aspects that may facilitate better understandingof the disclosure. Accordingly, the statements of this section are to beread in this light and are not to be understood as admissions about whatis in the prior art or what is not in the prior art.

In wireless communication systems, there is a requirement for highspectrum efficiency (SE) and seamless coverage. Multiple input multipleoutput (MIMO) techniques have been known as an effective way forimproving SE and coverage in wireless communication systems. Forexample, MIMO has been adopted as a key feature in Long Term Evolution(LTE)/LTE-Advanced (LTE-A) system developed by the third generationproject partnership (3GPP). To further enhance coverage and throughputfor some hot-spot and/or indoor massive user access scenarios, such asuniversity, Central Business District (CBD), subway station, stadium,air-port, railway station waiting-room and so on, a distributed MIMO(D-MIMO) feature has been proposed.

In a D-MIMO scenario, antennas may be located at different sites. Forinstance, a DAS may add remote radio units (RRUs) with multiple antennasto enhance coverage and capacity of a cellular system. DAS works byconnecting the RRUs to a processing unit of a base station via a highbandwidth and low latency link such as a Common Public Radio Interface(CPRI). The cellular system can make use of the RRUs in DAS to providemore uniform coverage, reduced outage, and higher throughputs,especially in shadowed and indoor locations. As an example, in LTE,D-MIMO can be applied in downlink to mitigate co-channel interference orincrease spatial multiplexing gain effectively, and consequently cellcapacity, average throughput and cell-edge user throughput can beimproved.

In a DAS system where multiple RRUs are distributed at different sites,joint pre-coding or beam-forming can be done to keep co-phase combiningat the receiver side. In addition, DAS may be combined with single user(SU) or paired multi-user (MU) MIMO communication by treating the RRUsas a distributed antenna array and/or equipping RRUs with multipleantennas. A preferred operating mode in MIMO systems may changedepending on interference conditions. In SU DAS, antenna selection (AS)is preferred over beamforming (BF) in terms of ergodic rate whenout-of-cell interference is incorporated into the communication model.In MU MIMO DAS, in one configuration, BF to a plurality of users ispreferred over BF to only the best user.

In order to minimize phase (and amplitude) difference among multipletransceiver chains so as to guarantee precise BF/pre-coding and obtainlarge diversity or multiplexing gain, antenna calibration is required.

SUMMARY

Antenna calibration for a distributed antenna system is a challenge forD-MIMO implementation. Methods, apparatuses and computer programproducts have been provided in the present disclosure for facilitatingantenna calibration with low complexity. It will be appreciated thatthough some embodiments of the present disclosure are described withreference to a LTE, LTE-A or 5G system, embodiments are not limitedthereto, but could be more widely applied to any wireless communicationsystem where similar problems exist.

Various embodiments of the present disclosure mainly aim at providingmethods, apparatuses and computer program products for antennacalibration for distributed antennas. Other features and advantages ofembodiments of the present disclosure will be understood from thefollowing description of various embodiments when read in conjunctionwith the accompanying drawings, which illustrate, by way of example, theprinciples of embodiments of the present disclosure.

In a first aspect of the disclosure, there is provided a method in adigital unit. The digital unit is connected, directly or indirectly,with a plurality of distributed antennas including (but not limited to)a first antenna, a second antenna and a third antenna. The methodcomprises: causing transmitting a first signal from the first antenna, asecond signal from the second antenna, and a third signal from the thirdantenna in a same frequency resource; obtaining a receiver andtransmitter side loop-back phase difference between the first antennaand the second antenna based on the first signal received at the thirdantenna, the second signal received at the third antenna, the thirdsignal received at the first antenna, and the third signal received atthe second antenna; and obtaining estimations of a time delay differenceand an initial phase difference between the first and second antennasbased on the obtained receiver and transmitter side loop-back phasedifference.

In an embodiment, the digital unit may cause transmitting the first,second and third signals in a time period during which no data istransmitted. In a further embodiment, the time period may include atleast one of: a Guard Period (GP) in a time divisional duplexing (TDD)frame structure, and a subframe during which no data and controlsignaling is transmitted.

In another embodiment, the digital unit may cause transmitting thefirst, second and third signals in a time divisional multiplexing (TDM)manner.

In an embodiment, the first, second and third signals transmitted fromthe first, second, and third antennas may be generated based on a sameconstant amplitude sequence.

In another embodiment, the digital unit may obtain the receiver andtransmitter side loop-back phase difference between the first and secondantennas by: obtaining a first loop-back phase for the first antenna bycomparing a phase for the third signal received at the first antenna anda phase for the first signal received at the third antenna; obtaining asecond loop-back phase for the second antenna by comparing a phase forthe third signal received at the second antenna and a phase for thesecond signal received at the third antenna; and obtaining the receiverand transmitter side loop-back phase difference by determining adifference between the first and second loop-back phases.

In some embodiments, the digital unit may obtain the estimations of thetime delay difference and the initial phase difference between the firstand second antennas by: estimating the time delay difference and theinitial phase difference according to polynomial fit based on a LeastSquare (LS) criterion or a Recursive Least Square criterion.

In an embodiment, the digital unit may obtain the receiver andtransmitter side loop-back phase difference between the first and secondantennas by: obtaining the receiver and transmitter side loop-back phasedifference between the first and second antennas for a subcarrier in thefrequency resource.

In another embodiment, the method may further comprise obtaining aloop-back amplitude difference between the first and second antennasbased on the first signal received at the third antenna, the secondsignal received at the third antenna, the third signal received at thefirst antenna and the third signal received at the second antenna. In afurther embodiment, the digital unit may obtain the loop-back amplitudedifference between the first antenna and the second antenna by:obtaining the loop-back amplitude difference between the first andsecond antennas for a subcarrier in the frequency resource.

In some embodiments, at least two of the first, second and thirdantennas may locate at different remote radio units (RRUs) connectedwith the digital unit.

In still another embodiment, the method may further comprise: adjustinga phase or timing of a signal to be transmitted from the first or secondantenna based on the estimation of the time delay difference or initialphase difference.

In a second aspect of the disclosure, there is provided an apparatusconnected, directly or indirectly, with a plurality of distributedantennas including (at least) a first antenna, a second antenna and athird antenna. The apparatus comprises: a transmission controlling unitconfigured to cause transmitting a first signal from the first antenna,a second signal from the second antenna, and a third signal from thethird antenna in a same frequency resource; a obtaining unit configuredto obtain a receiver and transmitter side loop-back phase differencebetween the first antenna and the second antenna based on the firstsignal received at the third antenna, the second signal received at thethird antenna, the third signal received at the first antenna, and thethird signal received at the second antenna; and an estimating unitconfigured to obtain estimations of a time delay difference and aninitial phase difference between the first and second antennas based onthe obtained receiver and transmitter side loop-back phase difference.

In a third aspect of the disclosure, there is provided an apparatusconnected, directly or indirectly, with a plurality of distributedantennas including (at least) a first antenna, a second antenna and athird antenna. The apparatus comprises a processor and a memory, thememory containing instructions executable by the processor whereby theapparatus is operative to: cause transmitting a first signal from thefirst antenna, a second signal from the second antenna, and a thirdsignal from the third antenna in a same frequency resource; obtain areceiver and transmitter side loop-back phase difference between thefirst antenna and the second antenna based on the first signal receivedat the third antenna, the second signal received at the third antenna,the third signal received at the first antenna, and the third signalreceived at the second antenna; and obtain estimations of a time delaydifference and an initial phase difference between the first and secondantennas based on the obtained receiver and transmitter side loop-backphase difference.

In a fourth aspect of the disclosure, there is provided a computerprogram product, comprising instructions which, when executed on one ormore processors, cause the one or more processors to carry out a methodaccording to the first aspect of the present disclosure.

In a fifth aspect of the disclosure, there is provided an apparatus in adigital unit connected, directly or indirectly, with a plurality ofdistributed antennas including (at least) a first antenna, a secondantenna and a third antenna. The apparatus comprises processing meansadapted to perform a method according the first aspect of the presentdisclosure.

In a sixth aspect of the disclosure, there is provided a computerreadable storage medium having a computer program product embodiedthereon. The computer program product includes instructions which, whenexecuted on at least one processor, cause the at least one processor tocarry out a method according the first aspect of the present disclosure.

In a seventh aspect of the disclosure, there is provided a digital unitcomprising the apparatus according to the second or third aspect of thedisclosure.

In an eighth aspect of the disclosure, there is provided a networkdevice comprising the apparatus according to the second or third aspectof the disclosure.

According to various aspects and embodiments as mentioned above, antennacalibration with low complexity are enabled for distributed antennasystems without requiring presence of a couple control unit (CCU).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and benefits of variousembodiments of the present disclosure will become more fully apparentfrom the following detailed description with reference to theaccompanying drawings, in which like reference numerals or letters areused to designate like or equivalent elements. The drawings areillustrated for facilitating better understanding of the embodiments ofthe disclosure and not necessarily drawn to scale, in which:

FIG. 1 illustrates an example wireless communication network in whichembodiments of the present disclosure may be implemented;

FIGS. 2A-2B show communication scenarios with DAS according toembodiments of the present disclosure;

FIG. 3 illustrates an antenna calibration solution based on an assistantnode;

FIG. 4 illustrates an antenna calibration solution based on CCU forco-site antenna arrays;

FIG. 5 illustrates a schematic block diagram of a calibration apparatusaccording to an embodiment of the present disclosure;

FIGS. 6A-6B illustrate flow charts of antenna calibration methodsaccording to embodiments of the present disclosure;

FIG. 7 illustrates a flow chart of another antenna calibration methodaccording to an embodiment of the present disclosure;

FIG. 8 illustrates a frame structure for transmitting a calibrationsignal according to an embodiment of the present disclosure;

FIG. 9 illustrates a flow chart of still another antenna calibrationmethod according to an embodiment of the present disclosure;

FIG. 10 illustrates a schematic block diagram of an apparatusimplemented as/in a digital unit according to an embodiment of thepresent disclosure; and

FIG. 11 illustrates a simplified block diagram of an apparatus that maybe embodied as/in a digital unit.

DETAILED DESCRIPTION

Hereinafter, the principle and spirit of the present disclosure will bedescribed with reference to illustrative embodiments. It should beunderstood that all these embodiments are given merely for one skilledin the art to better understand and further practice the presentdisclosure, but not for limiting the scope of the present disclosure.For example, features illustrated or described as part of one embodimentmay be used with another embodiment to yield still a further embodiment.In the interest of clarity, not all features of an actual implementationare described in this specification.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” and the like indicate that the embodimentdescribed may include a particular feature, structure, orcharacteristic, but it is not necessary that every embodiment includesthe particular feature, structure, or characteristic. Moreover, suchphrases are not necessarily referring to the same embodiment. Further,when a particular feature, structure, or characteristic is described inconnection with an embodiment, it is submitted that it is within theknowledge of one skilled in the art to affect such feature, structure,or characteristic in connection with other embodiments whether or notexplicitly described.

It shall be understood that although the terms “first” and “second” etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first element could be termed asecond element, and similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed terms.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be liming of exampleembodiments. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises”, “comprising”, “has”, “having”, “includes” and/or“including”, when used herein, specify the presence of stated features,elements, and/or components etc., but do not preclude the presence oraddition of one or more other features, elements, components and/orcombinations thereof.

In the following description and claims, unless defined otherwise, alltechnical and scientific terms used herein have the same meaning ascommonly understood by one of ordinary skills in the art to which thisdisclosure belongs.

As used herein, the term “wireless communication network” refers to anetwork following any suitable wireless communication standards, such asNew Radio (NR), Long Term Evolution (LTE), LTE-Advanced (LTE-A),Wideband Code Division Multiple Access (WCDMA), High-Speed Packet Access(HSPA), and so on. Furthermore, the communications between networkdevices in the wireless communication network may be performed accordingto any suitable generation communication protocol, including, but notlimited to, Global System for Mobile Communications (GSM), UniversalMobile Telecommunications System (UMTS), Long Term Evolution (LTE),and/or other suitable communication protocols, such as the firstgeneration (1G), the second generation (2G), 2.5G, 2.75G, the thirdgeneration (3G), the fourth generation (4G), 4.5G, the fifth generation(5G) communication protocols, wireless local area network (WLAN)standards, such as the IEEE 802.11 standards; and/or any otherappropriate wireless communication standard, and/or any other protocolseither currently known or to be developed in the future.

As used herein, the term “network device” refers to a device in awireless communication network via which a terminal device accesses thenetwork and receives services therefrom. The network device may refer toa base station (BS) or an access point (AP), for example, a node B(NodeB or NB), an evolved NodeB (eNodeB or eNB), a NR B (also referredto as a gNB), a Remote Radio Unit (RRU), a radio header (RH), a remoteradio head (RRH), a relay, a low power node such as a femto, a pico, andso forth, depending on the applied terminology and technology.

The term “terminal device” refers to any end device that may be capableof wireless communications. By way of example and not limitation, aterminal device may be referred to as user equipment (UE), a SubscriberStation (SS), a Portable Subscriber Station, a Mobile Station (MS), oran Access Terminal (AT). The terminal device may include, but notlimited to, a mobile phone, a cellular phone, a smart phone, voice overIP (VoIP) phones, wireless local loop phones, a tablet, a wearableterminal device, a personal digital assistant (PDA), portable computers,desktop computer, image capture terminal devices such as digitalcameras, gaming terminal devices, music storage and playback appliances,vehicle-mounted wireless terminal devices, wireless endpoints, mobilestations, laptop-embedded equipment (LEE), laptop-mounted equipment(LME), USB dongles, smart devices, wireless customer-premises equipment(CPE) and the like. In the following description, the terms “terminaldevice”, “terminal”, “user equipment” and “UE” may be usedinterchangeably.

As yet another example, in an Internet of Things (IOT) scenario, aterminal device may represent a machine or other device that performsmonitoring and/or measurements, and transmits the results of suchmonitoring and/or measurements to another terminal device and/or networkequipment. The terminal device may in this case be a machine-to-machine(M2M) device, which may in a 3GPP context be referred to as amachine-type communication (MTC) device. As one particular example, theterminal device may be a UE implementing the 3GPP narrow band internetof things (NB-IoT) standard. Examples of such machines or devices aresensors, metering devices such as power meters, industrial machinery, orhome or personal appliances, for example refrigerators, televisions,personal wearables such as watches etc. In other scenarios, a terminaldevice may represent a vehicle or other equipment that is capable ofmonitoring and/or reporting on its operational status or other functionsassociated with its operation.

As used herein, a DL transmission refers to a transmission from thenetwork device to a terminal device, and an UL transmission refers to atransmission in an opposite direction.

FIG. 1 illustrates an example wireless communication network 100 inwhich embodiments of the present disclosure may be implemented. As shownin FIG. 1, the wireless communication network 100 may include one ormore network devices (also referred to as network nodes), for example, anetwork device 101, which may be in a form of an eNB or gNB. It will beappreciated that the network device 101 can also be in a form of basestation (BS), a Node B (NB), a virtual BS, Base Transceiver Station(BTS), and/or Base Station Subsystem (BSS), AP and the like. The networkdevice 101 may be equipped with a plurality of distributed antennas(e.g., antennas 111-116) locating at different sites, for examplelocating at a plurality of RRUs 121-123. Using one or more of thedistributed antennas, the network device 101 provides radio connectivityto a set of terminal devices or UEs 102-1, 102-2, . . . , 102-N, whichare collectively referred to as “terminal device(s) 102”, within itscoverage, where N is a natural number. For example, DL data from thenetwork device 101 may be transmitted by joint beam-forming orpre-coding among the plurality of RRUs 121-123 to enable in-phasecombining at the terminal device 102 side. In other words, service datamay be jointly transmitted to a target terminal device(s) from severalsites, and joint beam-forming or pre-coding among a plurality ofantennas may be performed to enable in-phase combining at the receiverside. The network device 101 may also comprise or be equipped (orassociated) with a digital unit (DU) not shown in FIG. 1 which isresponsible for data and control signal processing.

Using a beam-forming or pre-coding technique, a network device is ableto adaptively optimize the radiation beam pattern of the DAS to achievehigh gain and controlled beam-width in desired directions by weightingantenna arrays in terms of spatial channel correlation. In this way,signal to interference plus noise ratio (SINR) or signal to noise ratio(SNR) for a desired user is maximized while interference to undesiredusers is minimized Consequently, inter-cell and intra-cell co-channelinterference is suppressed, and cell edge throughput and system capacityis improved.

Beam-forming weights are generated based on characteristic of a compoundspatial channel which is a combination of a spatial wireless channel andan antenna apparatus internal chain. In TDD wireless communicationsystems like TD-SCDMA, TD-LTE, and TD LTE-A, the spatial wirelesschannel has good reciprocity in uplink and downlink. It means that it ispossible for the network side to obtain channel status of a downlinkwireless channel by detecting an uplink wireless channel. But thereciprocity does not exist in internal chains of an antenna apparatus .An internal chain of an antenna apparatus usually consists ofintermediate frequency (IF) chain(s), radio frequency (RF) circuit andpossibly of a feeder cable connection between RRU and antennas, whichmay be different for different antennas, and may cause differentamplitude degradation and phase shifts. Besides, the amplitudeattenuation and phase shift property may vary with temperature,atmospheric humidity and device aging. Therefore, antennas calibrationis required to compensate the difference in amplitude and phase shiftsamong different antenna chains in order to keep channel reciprocity inuplink and downlink. It should be appreciated that even in a FDD system,antenna calibration is also required to facilitate accurate BFweighting/precoding.

FIG. 2A shows a communication scenario with DAS, in which a plurality ofdistributed antennas 201-210 are connected, directly or indirectly, to adigital unit (DU) 220 which may be a part of a base station or shared bya plurality of base stations. The plurality of antennas 201-210 may beused for data channel multiplexing and/or control channel combination.For example, antennas 201-204 may cover an area 231, antennas 205-207may cover an area 232 and antennas 208-210 may cover an area 233. Theareas 231-233 may be served by one or more base stations.

FIG. 2B shows another communication scenario with DAS, in which severaldistributed transmitting points (TPs) 241-248 are connected, directly orindirectly, to a DU 240. The plurality of TPs 241-248 may belong to oneor more base stations (not shown). In this example scenario, TPs 241-243serve UE 251, TPs 243-244 serve UE 252, and TP 245-248 serve UE 253,respectively.

In both example scenarios shown in FIGS. 2A-2B, it may be infeasible toapply centralized antenna array calibration with a couple network (e.g.,a Couple Control Unit (CCU)), since transmitters (e.g. antenna elements)are located at the different sites.

In addition, it has been realized by inventors of the present disclosurethat UL and DL channel reciprocity cannot be guaranteed by performingonly centralized antennas array calibration at each antenna site (i.e.,intra-transceiver antenna calibration) separately, since the internalchains of an antenna apparatus at distributed sites may be different.

Therefore, a new method and apparatus for antenna calibration in DASwithout a couple network is needed in order to minimize the amplitudeand phase difference among distributed antennas.

One way for distributed antennas calibration is to introduce anassistant node, such as a relay or UE, and such a method isschematically illustrated in FIG. 3. In this solution, the introducedassistant node 301 works as a reference for the distributed antennascalibration. In particular, all distributed antennas (e.g., 302 and 303)to be calibrated receive a calibration reference signal transmitted bythe assistant node 301 and calibrate their uplink antenna apparatuschains by referring to the calibration reference signal. The assistantnode 301 receives antenna calibration reference signals from thedistributed antennas 302 and 303 (which may be transmitted in timedivision multiplexing (TDM), code division multiplexing (CDM) or ahybrid multiplexing manner) and calibrates their downlink antennaapparatus chains by referring to the received signal. In such methods,the third-party assistant node 301 has to be introduced as a reference,and varying wireless channels among the assistant node 301 and targetdistributed antennas 302 and 303 may bring much uncertainty.Furthermore, since transceiver chain of the assistant node 301 isinvolved in the transceiver calibration chains, calibration result maybe inaccurate, and the method and apparatus for antenna calibrationbecome complex.

An alternative method for distributed antennas calibration isillustrated in FIG. 4. As shown in FIG. 4, calibration ports of acluster of RRUs 401 and 402 are connected by a combiner 410, and acalibration port 420 of the RRUs is used as a reference antenna port. Byusing a cable 430 instead of a direct wireless channel to feedbacksignals to the calibration port 420, and by joint detection of uplinkand downlink calibration signal, loop-back difference of uplink anddownlink difference among working radio paths are sounded andcompensated. This solution does not require a third-party assistantnode, and therefore calibration accuracy is improved and complexity isdecreased significantly. However, in a D-MIMO system, a cluster of RRUsassociated with a same DU (also referred to as a D-MIMO cluster) may bedistributed at different sites, and each RRU in the D-MIMO cluster mayconnect two-path or singe-path antennas without a CCU (403 and 404 shownin FIG. 4) by cable, and in such a scenario, the solution shown in FIG.4 can hardly be utilized since it requires that the cluster of RRUs 401and 402 be installed co-site. Furthermore, the external combiner 410involved in the solution may not be cost optimal.

Furthermore, inventors of the present disclosure have realized that atiny delay difference among antenna systems may cause a large phaseshift for a high frequency sub-carrier in OFDM systems. A field test hasshown that an error in a beam-forming pattern is often restricted to beless than 5 degrees by telecommunication operators. In other words, thedelay difference among antenna elements must be less than 1/32 Ts(sampling duration) for a 20 MHz TD-LTE system. None of the distributedantennas calibration solutions currently available can meet such strictrequirements on calibration accuracy with reasonable complexity.

In embodiments of the present disclosure, methods and apparatuses forantenna calibration in DAS without a couple network (e.g., a CCU) havebeen proposed. With embodiments of the present disclosure, calibrationaccuracy and system robustness for DAS may be improved with lowcomplexity. Embodiments of the present disclosure may be applied in aD-MIMO system based on multi-points joint pre-coding technology.

In a D-MIMO scenario, distributed antennas are located at the differentsites but a line of sight (LOS) path among them is always available. Forsimplicity, a calibration apparatus for three groups of antennas isillustrated schematically in FIG. 5, where one DU 501 connects withthree groups of RRUs 502-504 over, e.g. optical fiber 510-530respectively, and antenna radiation elements 541-546 are connected witha corresponding RRU (502, 503 or 504) by cable, for example. Antennacalibration for any of the antennas 541-546 may be controlled by the DU501. In some embodiments, both the DU 501 and antennas 541-546 can beconsidered as a part of a network device 500 which may be a base station(which may be a virtual concept). It should be appreciated that in someother embodiments, the network device 500 may include more or less RRUsor antennas or antenna arrays. Furthermore, in some scenarios, the DU501 may be shared by a plurality of network devices, and may beconnected, directly or indirectly, with antennas (or RRUs) associatedwith more than one network device.

FIG. 6 shows a flowchart of a method 600 in a digital unit according toan embodiment of the present disclosure. In an embodiment, distributedantenna calibration is divided into an initial calibration phase and aperiodical calibration phase, and the method 600 may be used for theinitial calibration phase and/or the periodical calibration phase.

The digital unit (DU) performing the method 600 is connected, directlyor indirectly, with a plurality of distributed antennas including, forexample, a first antenna, a second antenna and a third antenna. Forinstance, the DU may be a DU associated with the network device 101shown in FIG. 1, or the DU 501 shown in FIG. 5. For ease of discussion,the method 600 will be described below with reference to the DU 501 andthe scenario illustrated in FIG. 5. However, embodiments of the presentdisclosure are not limited thereto and may be applied more widely inother scenarios where similar problem exists.

As illustrated in FIG. 6, at block 610, the DU 501 causes transmissionsof a first signal from the first antenna, a second signal from thesecond antenna, and a third signal from the third antenna in a samefrequency resource. Each of the first, second and third signal may bereferred to as a calibration signal.

In an embodiment, prior to the operations of block 610, the DU 501 issynchronized to a Global Position System (GPS) or other absolute networkclocks and RRUs 502-504 recover timing from, for example, a CPRI whichinterconnects with the DU 501, when systems boot. The timing of CPRI maybe from the GPS.

In some embodiments, operations of block 610 may be triggered when RRUs502-504 are switched to a calibration status and receive an initialcalibration command after preheating of the RRUs 502-504 and DU 501.

Alternatively or in addition, operations of block 610 may be triggeredwhen RRUs 502-504 are switched to a periodical calibration status andreceive a periodical calibration command after initial calibration isdone.

Embodiments are not limited to any specific time and/or frequencyresources for transmitting the first, second, and third signals at block610. Just for illustration rather than limitation, in an embodiment, thefirst, second, and third signals may be transmitted in an empty timeslice, that is, in a time slice during which no data or control signalis transmitted. The empty time slice can be, for example, a Guard Period(GP) defined in a 3GPP TDD frame structure (such as LTE TDD framestructure) or an un-used sub-frame. Transmission in an empty time slicemay improve signal detection performance. In another embodiment, thefirst, second and third signal may be multiplexed with data traffic in anormal subframe.

As an example rather than limitation, in another embodiment, the first,second, and third signals may be transmitted in a TDM manner, whichhelps reduce interference to each other. It should be appreciated thatin a further embodiment, other multiplexing scheme such as CDM may beused instead. In the latter case where CDM is used, orthogonal codes arepreferably used for multiplexing the first, second and third signals.

The first, second, and third signals may be any predefined signal, andembodiments of the present disclosure are not limited to any specificway for constructing these calibration signals. For example, thecalibration signal may be constructed offline based on a constantamplitude sequence. In a LTE system, the calibration signal may beconstructed based on a root Zadoff-Chu (ZC) sequence as follows.

Assume that the u-th root ZC sequence defined by

${{x_{u}(n)} = e^{{- j}\; \frac{\pi \; {{un}{({n + 1})}}}{N_{zc}}}},$

0≤n≤N_(zc)−1 is used, N_(zc) is length of the sequence. A DiscreteFourier Transformation (DFT) operation may be applied to obtain acorresponding frequency domain ZC sequence x′_(u)(k)=DFT(x_(u)(n)), k=0,. . . , N_(zc)−1. By mapping x′_(u)(k) to one Orthogonal FrequencyDivision Multiplexing (OFDM) symbol, it is obtained x_(c)(k)=[0,x_(u)(1), . . . , x′_(u)(N₁), 0₁, . . . , 0_(N) ₂ , x′_(u)(N₁+1), . . ., x′_(u)(N_(ZC))],where N₁=N_(ZC)/2, and N₂=2048−N_(ZF) if 2048 pointsDFT/FFT is to be applied. After adding a pre-Cyclic Prefix (CP) and apost-CP, the transmitted signal s_(c)(n) in time domain iss_(c)(n)=[S_(OFDM)(N_(FFT)−N_(CP)+1, . . . , N_(FFT))S_(OFDM)(1, . . . ,N_(FFT))S_(OFDM)(1, . . . , N_(CP))], where S_(OFDM)(n)=DFT(x_(c)(k)),or S_(OFDM)(n)=FFT(x_(c)(k)).

In an embodiment, the CP length is N_(cp)=128, a length of the ZCsequence is N_(zc)=839, and length of DFT/FFT for x_(c)(k) isN_(FFT)=2048, however, it should be appreciated that embodiments of thepresent disclosure are not limited to any specific setting for thecalibration signal generation. In a Time Division-Synchronization CodeDivision Multiple Access (TD-SCDMA) system, similar calibration signalsmay be constructed.

For ease of discussion, it is assumed that the first antenna, the secondantenna, and the third antenna for transmitting the first, second andthird signals at block 610 are antennas 541, 542 and 543 shown in FIG.5, respectively. Though in this example, the first and second antennasbelong to a same RRU (i.e., RRU 502 shown in FIG. 5), embodiments arenot limited thereto. Further, in some embodiments, each of the firstantenna, the second antenna and the third antenna may be from a separateantenna array.

In an embodiment, at block 610, a calibration signal s1 (e.g.,s1=s_(c)(n)) is a sequence transmitted from the third antenna 543, thenat the first and second antennas, the received signal at subcarrier kmay be expressed as:

c _(1,k) e ^(jθ) ^(1,k) =H _(tx,3,k) *H _(rx,1,k) *G _(3,1,k) *s _(k)+n=c _(tx,3,k) *e ^(jθ) ^(tx,3,k) *c _(rx,1) *e ^(jθ) ^(rx,1,k) *G_(3,1,k) *s _(k) +n  (1)

c _(2,k) e ^(jθ) ^(2,k) =H _(tx,3,k) *H _(rx,2,k) *G _(3,2,k) *s _(k)+n=c _(tx,3,k) *e ^(jθ) ^(tx,3,k) *c _(rx,2) *e ^(jθ) ^(rx,2,k) *G_(3,2,k) *s _(k) +n  (2)

Here, H_(tx,n,k) and H_(rx,m,k) denote frequency domain response of then-th serving transmitting path and the m-th serving receiving path atthe k-th subcarrier, respectively. For example, H_(tx,3,k) denotesfrequency domain response of the third antenna transmitting path at thek-th subcarrier. G_(m,n,k) denotes frequency domain channel response ofthe wireless connection between antenna m and antenna n for the k-thsubcarrier, s_(k) denotes the calibration signal transmitted in the k-thsubcarrier; n_(k) denotes white noise at the k-th subcarrier. C_(m,k)denotes amplitude of the signal received at the m-th antenna and thek-th subcarrier, and θ_(m,k) denotes phase of the signal received at them-th antenna and the k-th subcarrier. For convenience, the subscript kmay be omitted.

Likewise, at block 610, the calibration signal s_(k) is transmitted fromthe first antenna 541 at subcarrier k, and the signal received by theantenna 543 may be expressed as:

c _(3,k) e ^(jθ) ^(1,k) =H _(tx,1,k) *H _(rx,3,k) *G _(1,3,k) *s _(k)+n=c _(tx,1,k) *e ^(jθ) ^(tx,1,k) *c _(rx,3,k) *e ^(jθ) ^(rx,3,k) *G_(1,3,k) *s _(k) +n   (3)

Further, at block 610, the calibration signal s_(k is) also transmittedby the second antenna 542 at subcarrier k, and at the antenna 543, thereceived signal may be expressed as:

c _(4,k) e ^(jθ) ^(4,k) =H _(tx,2,k) *H _(rx,3,k) *G _(2,3,k) *s _(k)+n=c _(tx,2,k) *e ^(jθ) ^(tx,2,k) *c _(rx,3,k) *e ^(jθ) ^(rx,3,k) *G_(2,3,k) *s _(k) +n   (4).

Based on the first signal received at the third antenna 543(C_(3,k)e^(jθ) ^(3,k) ), the second signal received at the third antenna543 (C_(4,k)e^(jθ) ^(4,k) ), the third signal received at the firstantenna 541 (C_(1,k)e^(jθ) ^(1,k) ), and the third signal received atthe second antenna 542 (C_(2,k)e^(jθ) ^(2,k) ), at block 620, the DU 501obtains a receiver and transmitter side loop-back phase differenceϕ_(1,2) between the first antenna 541 and the second antenna 542.

Here, “loop-back” refers to a combination of uplink and downlink (or, atransmitting link and a receiving link) for a specified path, and“receiver and transmitter side loop-back phase” (also referred to asloop-back phase) is a phase difference between a signal transmitted froma specified antenna and a signal received at the same specified antenna.

At block 620, the DU 501 may obtain the receiver and transmitter sideloop-back phase difference ϕ_(1,2) between the first and second antennas541 and 542 via any suitable computation, and embodiments are notlimited to any specific algorithm for obtaining the difference. Just forillustration purpose, an example implementation 620′ of the block 620 isprovided in FIG. 6B. In this example, the DU 501 obtains a firstloop-back phase for the first antenna 541 at block 621 by comparing aphase for the third signal received at the first antenna (i.e., thesignal C_(1,k)e^(jθ) ^(1,k) ), and a phase for the first signal receivedat the third antenna (i.e., the signal C_(3,k)e^(jθ) ^(3,k) ).

It is assumed that the wireless air-interface channel from antenna m toantenna n is same as the channel from antenna n to antenna m, or inother words, G_(m,n)=G_(n,m), then at block 621 the first loop-backphase for the first antenna 541 at subcarrier k may be obtained to be:ϕ_(1,k)=θ_(tx,1,k)−θ_(rx,1,k)=θ_(1,k)−θ_(3,k).

Likewise, at block 622, the DU 501 obtains a second loop-back phase forthe second antenna 542 by comparing a phase for the third signalreceived at the second antenna (i.e., the signal C_(2,k)e^(jθ) ^(2,k) ),and a phase for the second signal received at the third antenna (i.e.,the signal C_(4,k)e^(jθ) ^(4,k) ), i.e.,ϕ_(2,k)=θ_(tx,2,k)−θ_(rx,2,k)=θ_(2,k)−θ_(4,k).

At block 623, the DU 501 obtains the receiver and transmitter sideloop-back phase difference between the first antenna 541 and the secondantenna 542 by determining a difference between the first and secondloop-back phases ϕ_(1,k) and ϕ_(2,k). That is, the DU 501 obtains:

ϕ_(1,2,k)=ϕ_(1,k)−ϕ_(2,k)=(θ_(1,k)−θ_(3,k))−(θ_(2,k)−θ_(4,k))  (5)

In an embodiment, the DU 501 may obtain a loop-back phase differencebetween the first antenna 541 and the second antenna 542 for eachsubcarrier k used for transmitting the calibration signal, for exampleby using the example implementation 620′. In another embodiment, the DU501 may obtain a loop-back phase difference between the first antenna541 and the second antenna 542 for only a subset of the subcarriers usedfor transmitting the calibration signal.

Now referring back to FIG. 6A. At block 630, the DU 501 obtainsestimations of a time delay difference and an initial phase differencebetween the first and second antennas 541 and 542 based on the obtainedloop-back phase difference ϕ_(1,2).

Assuming that the time delay between the first and second antennas 541and 542 is Δ_(t) and the initial phase difference is φ_(ini), theloop-back phase difference ϕ_(1,2,k) for subcarrier k, can be writtenas:

$\begin{matrix}{\phi_{1,2,k} = {{\frac{2\; \pi}{N} \times k \times \Delta \; {t/T_{s}}} + \phi_{ini} + {n_{k}.}}} & (6)\end{matrix}$

Therefore, at block 630, the DU 501 may obtain estimations for Δ_(t) andφ_(ini) based on equation (6) and the ϕ_(1,2,k) obtained at block 620 or620′.

Embodiments are not limited to any specific algorithm for obtaining theestimation for Δ_(t) and φ_(ini) based on ϕ_(1,2,k). As an example,estimations for Δ_(t) and φ_(ini) may be obtained by Least Square (LS)polynomial fit on loop-back phase difference ϕ_(1,2,k) for sub-carrierk. That is, at block 630, the DU 501 may obtain Δt_(est) and φ_(ini)_(_) _(est) as follows:

$\begin{matrix}{{\Delta \; t_{est}} = {\frac{{L \cdot {\sum\limits_{k \in K}\left( {k \cdot \phi_{1,2,k}} \right)}} - {\sum\limits_{k \in K}{\phi_{1,2,k} \cdot {\sum\limits_{k \in K}k}}}}{{L \cdot {\sum\limits_{k \in K}k^{2}}} - \left( {\sum\limits_{k \in K}k} \right)^{2}}*\frac{N}{2\; \pi}}} & (7) \\{\phi_{{ini}\; \_ \; {est}} = {\frac{{\sum\limits_{k \in K}{\left( {k \cdot \phi_{1,2,k}} \right) \cdot {\sum\limits_{k \in K}k}}} - {\sum\limits_{k \in K}{\phi_{1,2,k} \cdot {\sum\limits_{k \in K}k^{2}}}}}{\left( {\sum\limits_{k \in K}k} \right)^{2} - {L \cdot {\sum\limits_{k \in K}k^{2}}}}.}} & (8)\end{matrix}$

Δt_(est) and φ_(ini) _(_) _(est) denote the estimations for Δ_(t) andφ_(ini), respectively, K is a set (or subset) of sub-carriers fortransmitting the calibration signal, and L is size of the set of K.

In another embodiment, at block 630, a polynomial fit based on aRecursive Least Square criterion may be utilized instead for obtainingthe estimations.

In some embodiments, besides the loop-back phase difference, the DU 501may further obtain a loop-back amplitude difference between the firstand second antennas 541 and 542, for example, based on the first signalreceived at the third antenna, the second signal received at the thirdantenna, the third signal received at the first antenna and the thirdsignal received at the second antenna, as shown in block 640 of FIG. 6A.In particular, in an embodiment, the DU 501 may obtain the loop-backamplitude difference between the first and second antennas for some orall subcarriers in the frequency resource used for transmitting thecalibration signal. As an example, the loop-back amplitude difference atsubcarrier k between the first and second antennas 541 and 542 may beexpressed by equation (9):

A _(1,2,k)=(c _(rx,2,k) /c _(tx,2,k))/(c _(rx,1,k) /c _(tx,1,k))=(c_(1,k) /c _(4,k))·(c _(3,k) /c _(2,k))  (9).

Though antenna calibrations are described with reference to the firstantenna 541 and the second antenna 542, it should be appreciated thatsimilar method can be applied for calibrating any other antennas. Forexample, calibration for antennas 544 and 545 may be performed inparallel with the calibration for antennas 541 and 542, by using thesame method 600. In this case, there may be signals transmittedsimultaneously from the antenna 545 (or 544) and the antenna 541 (or542), and the simultaneously transmission may be separated in frequencydomain or in code domain by using orthogonal code sequences. In anotherembodiment, calibration for different antennas may be performed inseries.

The obtained estimations for Δ_(t) and φ_(ini) (and A_(1,2,k)) enableadjustment of signals to be transmitted to compensate the phase (andamplitude) difference between antennas, such that calibration can beachieved. The adjustment of the signals to be transmitted based on theestimation of the time delay difference or initial phase difference maybe performed in any suitable manner, for example, with any methodcurrently available, and embodiments of the present disclosure are notlimited to any specific way for the adjustment.

Optionally, as shown in FIG. 6A, at block 650, the DU 501 may adjust aphase or timing of a signal to be transmitted from the first or secondantenna (541 or 542) based on the estimation of the time delaydifference or initial phase difference.

In an embodiment, at block 650, the DU 501 may choose one phase (and/ortime delay) difference as baseline or reference, and compensate thephase (and/or time delay) difference of the other antennas according tothe baseline/reference.

The method 600 enables antenna calibration for DAS without introducingany third-party absolute reference or an assistant node, and does notneed information exchange with other network devices. Furthermore, a CCUfor calibration can be avoided, which results in complexity and costreduction.

In the method 600, the calibration signal is transmitted overair-interface and the loop-back difference of uplink and downlinkphase/delay among working paths are sounded and compensated jointly,which improves distributed antennas system calibration accuracy andsystem robustness and at the same time decreases the calculationcomplexity.

FIG. 7 shows a flowchart of another method 700 in a digital unitaccording to an embodiment of the present disclosure. The DU isconnected, directly or indirectly, with a plurality of distributedantennas including (but not limited to) antennas A, B and C. Forinstance, the DU may be the DU 501 shown in FIG. 5. For ease ofdiscussion, the method 700 will be described below with reference to theDU 501 and the scenario illustrated in FIG. 5. However, embodiments ofthe present disclosure are not limited thereto and may be applied morewidely in other scenarios where similar problem exists.

As illustrated in FIG. 7, at block 710, the DU 501 constructs acalibration signal, for example, the signal s_(c)(n) described withreference to method 600. At block 720, the DU 501 determines whether acalibration flag is on. If the calibration flag is on, it means thatcalibration is triggered or activated, and the DU 501 performs antennacalibration for antennas A and B at blocks 730-750; otherwise,calibration is deactivated, and the process ends at block 760. As shownin FIG. 7, at block 730, the DU 501 causes transmitting the calibrationsignal from antenna C, and the calibration signal is received by antennaA and B.

Note that the calibration signal may be transmitted in a normal subframeor an empty time slice. FIG. 8 shows an example for transmitting thecalibration signal in a GP of a special subframe 800 of a LTE TDDsystem. As shown in FIG. 8, the special subframe 800 includes a DownlinkPilot Time Slot (DwPTS) 801 for DL transmission, a Uplink Pilot TimeSlot (UpPTS) 802 for uplink transmission and a GP 803. In this example,the calibration signal consisting of a pre-CP 823, a calibrationsequence 833 and a post-CP 843 is transmitted in an OFDM symbol in theGP 803, while an antenna calibration command (AC-Command) is transmittedat a time slice 813 in the GP 803. The calibration flag triggering theantenna calibration at block 720 of FIG. 7 may be included in theAC-Command

Now referring back to FIG. 7. At block 740, the DU 501 causestransmitting the calibration signal from antennas A and B, and thecalibration signals are received by the antenna C. At block 750, the DU501 obtains a phase difference between the antennas A and B for somesubcarriers based on the received signals at antennas A, B and C, forexample by solving the equations (1)-(4). Furthermore, time delaydifference and initial phase may be obtained based on the phasedifference, for example by using LS linear fitting. The time delay orphase difference may be compensated by the the DU 501 using any suitablemethod based on the obtained time delay or phase difference.

FIG. 9 shows a flowchart of still another method 900 in a DU accordingto an embodiment of the present disclosure. The DU is connected,directly or indirectly, with a plurality of distributed antennas. Forinstance, the DU may be the DU 501 shown in FIG. 5. For ease ofdiscussion, the method 900 will be described below with reference to theDU 501 and the scenario illustrated in FIG. 5. However, embodiments ofthe present disclosure are not limited thereto and may be applied morewidely in other scenarios where similar problem exists.

As illustrated in FIG. 9, at block 910, the DU 501 constructs acalibration signal, for example, the signal s_(c)(n) described withreference to method 600. At block 920, the DU 501 determines whetherinitial antenna calibration is triggered. Distributed antenna systemcalibration is performed at block 930 if the initial antenna calibrationis triggered; otherwise, the process ends at block 960. Note that atblock 930, any embodiment of method 600 or 700 may be applied forachieving the antenna calibration. After the initial antennacalibration, at block 940, the DU 501 determines whether periodicantenna calibration is triggered. If so, periodic antenna calibrationmay be performed at block 950, for example by applying an implementationof method 600 or 700; otherwise, the process ends at block 960.

Compared with a conventional antenna calibration method, embodiments ofthe present invention enable distributed antennas calibration, forexample for TDD systems based on D-MIMO technology. With embodiments ofthe present disclosure, antenna calibration may be implemented withoutintroducing any absolute reference and without relying on the CCU. Thecalibration signal goes through wireless air-interface. Some embodimentsmay keep the reciprocity of uplink and downlink channel in TDD systemswell, and thereby further enhance the joint pre-coding transmissionperformance of D-MIMO systems.

FIG. 10 illustrates a schematic block diagram of an apparatus 1000 in awireless communication network, for example, the wireless communicationnetwork 100 shown in FIG. 1. The apparatus 1000 may be implemented as orin a DU 501 shown in FIG. 5. The DU 501 is connected, directly orindirectly, with a plurality of antennas including (but not limited to)a first antenna, a second antenna and a third antenna. The apparatus1000 is operable to carry out any of the example methods 600, 700 and900 described with reference to FIGS. 6-9 and possibly any otherprocesses or methods. It is also to be understood that the methods 600,700, 900 are not necessarily carried out by the apparatus 1000. Someoperations of the methods 600, 700, 900 can be performed by one or moreother entities.

As illustrated in FIG. 10, the apparatus 1000 includes a transmissioncontrolling unit 1001, an obtaining unit 1002, and an estimating unit1003. The transmission controlling unit 1001 is configured to causetransmitting a first signal from the first antenna, a second signal fromthe second antenna, and a third signal from the third antenna in a samefrequency resource. The obtaining unit 1002 is configured to obtain areceiver and transmitter side loop-back phase difference between thefirst antenna and the second antenna based on the first signal receivedat the third antenna, the second signal received at the third antenna,the third signal received at the first antenna, and the third signalreceived at the second antenna. The estimating unit 1003 is configuredto obtain estimations of a time delay difference and an initial phasedifference between the first and second antennas based on the obtainedloop-back phase difference.

Optionally, in some embodiments, the apparatus 1000 may further includea amplitude difference obtaining unit 1012, configured to obtain aloop-back amplitude difference between the first and second antennasbased on the first signal received at the third antenna, the secondsignal received at the third antenna, the third signal received at thefirst antenna and the third signal received at the second antenna.

Alternatively or in addition, in another embodiment, the apparatus 1000may further include a compensating unit 1004, configured to adjust aphase or timing of a signal to be transmitted from the first or secondantenna based on the estimation of the time delay difference or initialphase difference.

In some embodiments, the transmission controlling unit 1001, theobtaining unit 1002, the estimating unit 1003, the amplitude differenceobtaining unit 1012 and the compensating unit 1004 may be configured toperform the operations of blocks 610-650 of FIG. 6A respectively, andtherefore relevant descriptions provided with reference to method 600and FIGS. 6A-6B also apply here and details will not be repeated.

FIG. 11 illustrates a simplified block diagram of an apparatus 1110 thatmay be embodied in or as a DU, for example, the DU 501 shown in FIG. 5

The apparatus 1110 may include one or more processors 1111, such as adata processor (DP) and one or more memories (MEM) 1112 coupled to theprocessor 1111. The apparatus 1110 may further include one or moretransmitters (TX)/receivers (RX) 1113 coupled to the processor 1111. TheMEM 1112 may be non-transitory machine readable storage medium and itmay store a program or a computer program product (PROG) 1114. The PROG1114 may include instructions that, when executed on the associatedprocessor 1111, enable the apparatus 1110 to operate in accordance withthe embodiments of the present disclosure, for example to perform themethod 600, 700 or 900. A combination of the one or more processors 1111and the one or more MEMs 1112 may form processing means 1115 adapted toimplement various embodiments of the present disclosure.

Various embodiments of the present disclosure may be implemented bycomputer program or computer program product executable by the processor1111, software, firmware, hardware or in a combination thereof.

The MEMs 1112 may be of any type suitable to the local technicalenvironment and may be implemented using any suitable data storagetechnology, such as semiconductor based memory terminal devices,magnetic memory terminal devices and systems, optical memory terminaldevices and systems, fixed memory and removable memory, as non-limitingexamples.

The processor 1111 may be of any type suitable to the local technicalenvironment, and may include one or more of general purpose computers,special purpose computers, microprocessors, digital signal processorsDSPs and processors based on multicore processor architecture, asnon-limiting examples.

Although some of the above description is made in the context of awireless communication system shown in FIG. 1, it should not beconstrued as limiting the spirit and scope of the present disclosure.The principle and concept of the present disclosure may be moregenerally applicable to other scenarios.

In addition, the present disclosure may also provide computer readablestorage medium such as a memory containing the computer program orcomputer program product as mentioned above, which includesmachine-readable media and machine-readable transmission media. Themachine-readable media may also be called computer-readable media, andmay include machine-readable storage media, for example, magnetic disks,magnetic tape, optical disks, phase change memory, or an electronicmemory terminal device like a random access memory (RAM), read onlymemory (ROM), flash memory devices, CD-ROM, DVD, Blue-ray disc and thelike. The machine-readable transmission media may also be called acarrier, and may include, for example, electrical, optical, radio,acoustical or other form of propagated signals—such as carrier waves,infrared signals, and the like.

The techniques described herein may be implemented by various means sothat an apparatus implementing one or more functions of a correspondingapparatus described with an embodiment includes not only prior artmeans, but also means for implementing the one or more functions of thecorresponding apparatus described with the embodiment and it may includeseparate means for each separate function, or means that may beconfigured to perform two or more functions. For example, thesetechniques may be implemented in hardware (one or more apparatuses),firmware (one or more apparatuses), software (one or more modules), orcombinations thereof. For a firmware or software, implementation may bemade through modules (for example, procedures, functions, and so on)that perform the functions described herein.

Example embodiments herein have been described above with reference toblock diagrams and flowchart illustrations of methods and apparatuses.It will be understood that each block of the block diagrams andflowchart illustrations, and combinations of blocks in the blockdiagrams and flowchart illustrations, respectively, can be implementedby various means including hardware, software, firmware, and acombination thereof. For example, in one embodiment, each block of theblock diagrams and flowchart illustrations, and combinations of blocksin the block diagrams and flowchart illustrations can be implemented bycomputer program or computer program product which includes computerprogram instructions. These computer program instructions may be loadedonto a general purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions which execute on the computer or other programmabledata processing apparatus create means for implementing the functionsspecified in the flowchart block or blocks.

Further, while operations are depicted in a particular order, thisshould not be understood as requiring that such operations be performedin the particular order shown or in sequential order, or that allillustrated operations be performed, to achieve desirable results. Incertain circumstances, multitasking and parallel processing may beadvantageous. Likewise, while several specific implementation detailsare contained in the above discussions, these should not be construed aslimitations on the scope of the subject matter described herein, butrather as descriptions of features that may be specific to particularembodiments. Certain features that are described in this specificationin the context of separate embodiments can also be implemented incombination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesub-combination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

It will be obvious to a person skilled in the art that, as thetechnology advances, the inventive concept can be implemented in variousways. The above described embodiments are given for describing ratherthan limiting the disclosure, and it is to be understood thatmodifications and variations may be resorted to without departing fromthe spirit and scope of the disclosure as those skilled in the artreadily understand. Such modifications and variations are considered tobe within the scope of the disclosure and the appended claims. Theprotection scope of the disclosure is defined by the accompanyingclaims.

1. A method in a digital unit connected with a plurality of distributedantennas including a first antenna, a second antenna and a thirdantenna, the method comprising: causing transmitting a first signal fromthe first antenna, a second signal from the second antenna, and a thirdsignal from the third antenna in a same frequency resource; obtaining areceiver and transmitter side loop-back phase difference between thefirst antenna and the second antenna based on the first signal receivedat the third antenna, the second signal received at the third antenna,the third signal received at the first antenna, and the third signalreceived at the second antenna; and obtaining estimations of a timedelay difference and an initial phase difference between the first andsecond antennas based on the obtained receiver and transmitter sideloop-back phase difference.
 2. The method of claim 1, wherein thecausing transmitting comprises: causing transmitting the first, secondand third signals in a time period during which no data is transmitted.3. The method of claim 2, wherein the time period includes at least oneof: a Guard Period in a time divisional duplexing, TDD, frame structure,and a subframe during which no data and control signaling istransmitted.
 4. The method of claim 1, wherein the first, second andthird signals transmitted from the first, second, and third antennas aregenerated based on a same constant amplitude sequence.
 5. The method ofclaim 1, wherein the causing transmitting comprises: causingtransmitting the first signal, the second signal and the third signal ina time divisional multiplexing manner.
 6. The method of claim 1, whereinobtaining the receiver and transmitter side loop-back phase differencebetween the first and second antennas comprises: obtaining a firstloop-back phase for the first antenna by comparing a phase for the thirdsignal received at the first antenna and a phase for the first signalreceived at the third antenna; obtaining a second loop-back phase forthe second antenna by comparing a phase for the third signal received atthe second antenna and a phase for the second signal received at thethird antenna; and obtaining the receiver and transmitter side loop-backphase difference by determining a difference between the first andsecond loop-back phases.
 7. The method of claim 1, wherein obtaining theestimations of the time delay difference and the initial phasedifference between the first and second antennas comprise: estimatingthe time delay difference and the initial phase difference according topolynomial fit based on a Least Square criterion or a Recursive LeastSquare criterion.
 8. The method of claim 1, wherein the obtaining thereceiver and transmitter side loop-back phase difference between thefirst and second antennas comprises: obtaining the receiver andtransmitter side loop-back phase difference between the first and secondantennas for a subcarrier in the frequency resource.
 9. The method ofclaim 1, further comprising: obtaining a loop-back amplitude differencebetween the first and second antennas based on the first signal receivedat the third antenna, the second signal received at the third antenna,the third signal received at the first antenna and the third signalreceived at the second antenna.
 10. The method of claim 8, whereinobtaining the loop-back amplitude difference between the first antennaand the second antenna comprises: obtaining the loop-back amplitudedifference between the first and second antennas for a subcarrier in thefrequency resource.
 11. (canceled)
 12. (canceled)
 13. An apparatusconnected with a plurality of distributed antennas including a firstantenna, a second antenna and a third antenna, the apparatus comprisinga processor and a memory, said memory containing instructions executableby said processor whereby said apparatus is operative to: causetransmitting a first signal from the first antenna, a second signal fromthe second antenna, and a third signal from the third antenna in a samefrequency resource; obtain a receiver and transmitter side loop-backphase difference between the first antenna and the second antenna basedon the first signal received at the third antenna, the second signalreceived at the third antenna, the third signal received at the firstantenna, and the third signal received at the second antenna; and obtainestimations of a time delay difference and an initial phase differencebetween the first and second antennas based on the obtained receiver andtransmitter side loop-back phase difference.
 14. The apparatus of claim13, wherein said memory contains instructions executable by saidprocessor whereby said apparatus is further operative to causetransmitting the first, second and third signals in a time period duringwhich no data is transmitted.
 15. The apparatus of claim 14, wherein thetime period includes at least one of: a Guard Period in a timedivisional duplexing, TDD, frame structure, and a subframe during whichno data and control signaling is transmitted.
 16. The apparatus of claim13, wherein the first, second and third signals transmitted from thefirst, second, and third antennas are generated based on a same constantamplitude sequence.
 17. The apparatus of claim 13, wherein said memorycontains instructions executable by said processor whereby saidapparatus is further operative to cause transmitting the first, secondand third signals in a time divisional multiplexing manner.
 18. Theapparatus of claim 13, where said memory contains instructionsexecutable by said processor whereby said apparatus is further operativeto obtain the receiver and transmitter side loop-back phase differencebetween the first and second antennas by: obtaining a first loop-backphase for the first antenna by comparing a phase for the third signalreceived at the first antenna and a phase for the first signal receivedat the third antenna; obtaining a second loop-back phase for the secondantenna by comparing a phase for the third signal received at the secondantenna and a phase for the second signal received at the third antenna;and obtaining the receiver and transmitter side loop-back phasedifference by determining a difference between the first and secondloop-back phases.
 19. The apparatus of claim 13, wherein said memorycontains instructions executable by said processor whereby saidapparatus is further operative to obtain the estimations of a time delaydifference and an initial phase difference between the first and secondantennas comprise: estimating the time delay difference and the initialphase difference according to polynomial fit based on a Least Squarecriterion or a Recursive Least Square criterion.
 20. The apparatus ofclaim 13, wherein said memory contains instructions executable by saidprocessor whereby said apparatus is further operative to obtain thereceiver and transmitter side loop-back phase difference between thefirst and second antennas by: obtaining the receiver and transmitterside loop-back phase difference between the first and second antennasfor a subcarrier in the frequency resource.
 21. The apparatus of claim13, herein said memory contains instructions executable by saidprocessor whereby said apparatus is further operative to: obtain aloop-back amplitude difference between the first and second antennasbased on the first signal received at the third antenna, the secondsignal received at the third antenna, the third signal received at thefirst antenna and the third signal received at the second antenna. 22.The apparatus of claim 21, wherein said memory contains instructionsexecutable by said processor whereby said apparatus is further operativeto obtain the loop-back amplitude difference between the first antennaand the second antenna by: obtaining the loop-back amplitude differencebetween the first and second antennas for a subcarrier in the frequencyresource.
 23. (canceled)
 24. (canceled)
 25. (canceled)